(1) Field of the Invention
This invention relates to a protection circuit, and more particularly relates to a latch-control protection circuit.
(2) Description of the Prior Art
To prevent over-voltage, over-current, or other abnormal conditions from damaging the power converter and the load circuit, a protection circuit is usually used to restrict the level of the output voltage, the amount of the output current, or the operating temperature until the abnormal condition is passed away.
Take an over-voltage protection circuit for example, when the output voltage of the power converter detected by the over-voltage protection circuit is too high, the over-voltage protection circuit stops power supplied to the power converter or alters the power supply of the power converter to low-voltage one. Meanwhile, a silicon controlled rectifier (SCR) unit is usually demanded for the protection circuit to enable latch operation. That is, when an over-voltage condition is detected, the over-voltage protection circuit turns into the over-voltage protection mode. The over-voltage protection circuit should be kept at the over-voltage protection mode by using the SCR unit until the output voltage returned to normal.
FIG. 1 is a circuit diagram of a typical SCR unit 10 and the peripheral circuit. As shown, the SCR unit 10 is connected to a delay circuit 12, which is composed of a resistor R3 and a capacitor C1. A protecting event detecting signal DET received by the delay circuit 12 is utilized to turn on the switch Q1 to pull down the state signal STATE to the ground VSS. The state signal STATE is utilized to activate a judging circuit (not shown) to enable the protecting operation. After the protecting operation is enabled, the judging circuit would be latched by the grounded state signal STATE unless a bias signal VBIAS is generated to reset the SCR unit 10.
However, the usage of SCR unit 10 in the latch-control protection circuit has the following drawbacks. Firstly, the SCR unit 10 is expensive and the parameters thereof are hard to be precisely controlled by using the ordinary semiconductor fabrication process. In addition, the delay circuit 12 integrated to the SCR unit 10 usually needs a greater time constant to prevent misjudging for happening. Such drawbacks restrict the application of the protection circuit and increase the overall cost.